Traditionally, in a wireless communication system, an RBS transmits downlink, DL, signals to user equipments, UEs, by broadcasting the DL signals in the entire cell which is covered by the RBS. In other words, the energy of a DL signal is broadcasted in the entire cell independent of the location of the UE to which the DL signals are to be received, hereinafter called the receiving UE.
To be able to concentrate the energy of DL signals in a direction from the RBS towards the receiving UE, a method called beam forming is used. In beam forming, an RBS is equipped with multiple antennas. The antennas are individually supplied with a weighted amount of the DL signal in such a way that the individual signals, when transmitted from the RBS, experience constructive interference between each other at a transmission angle towards a receiving UE while at other angles the individual signals experience destructive interference. As a result, the overlapping total DL signal will be concentrated in the direction towards the receiving UE.
The advantages of beam forming are, among others, increased signal strength at the location of the receiving UE, and reduced average interference to other cells. These advantages come as a result of the energy concentration in the direction towards the receiving UE. As a result, the signal to interference ratio (SINR) at the receiving UE is improved.
However, one drawback with beam forming is that it creates high interference variations in neighbor cells, which has a negative impact on link adaption in the neighboring cells. This phenomenon is shown in FIG. 1, which shows two neighboring cells at three different, consecutive transmission time intervals, TTI: TTI1, TTI2, TTI3 in a possible transmission scenario. In the transmission scenario, a wireless network comprises a first RBS 20 providing coverage in a first cell 22, and a second RBS 30 providing coverage in a second cell 32. There are also a first UE 12 and a second UE 14 located in the first cell 22, and a UE 16 located in the second cell 32.
In TTI1, a DL signal from the first RBS 20 is formed and directed as a beam 42 towards the first UE 12 in the first cell 22. During TTI1 the UE 16 in the second cell 32 experiences no, or at least very low intercell interference from the first cell 22, since the UE 16 in the second cell 32 is not in a location where it is exposed to the beam 42 from the first RBS 20.
In the following TTI, TTI2, on the other hand, the direction of the beam 42 is pointing towards the second UE 14 in the first cell 22. During the TTI2, the UE 16 in the second cell 32 experiences high inter-cell interference from the first cell 22, since the UE 16 in the second cell is exposed to the beam 42 from the first RBS 20.
In the TTI3, following the TTI2, the direction of the beam 42 is now pointing back towards the first UE 12 again. Consequently, during TTI3 the UE 16 in the second cell again experiences no, or very low intercell interference from the first cell 22.
As shown, beam forming can create large interference variations in neighbor cells between consecutive TTIs. These interference variations make it difficult for the link adaption in the neighbor cells to select a suitable Modulation Coding Scheme, MCS, for the UEs, which may lead to a reduced user throughput.
Consequently, it would be advantageous to be able to reduce the interference variations in neighbor cells between consecutive TTIs, occurring when an RBS transmits DL signals using beam forming.